276 ISOTOPIC TRACERS AND NUCLEAR RADIATIONS [Chap. 8 



the connecting tube, a stirrer is lowered to the center of the tube and slowly 

 pulled up and down in the tube for about ten cycles. After standing about 

 48 hr, the gradient is sufficiently linear to be used in carrying out measure- 

 ments. The early papers on this method [15] recommended saturating the 

 kerosene-bromobenzene mixture with water at a suitable vapor pressure, 

 but in deuterium measurements this is undesirable because of exchange. 

 Although it is true that if the gradient is used exclusively for deuterium 

 analysis the various levels of the gradient do tend to become saturated with 

 the proper deuterium oxide-water ratios, exchange may still occur while the 

 drop is settling, and it is best to keep the gradient unsaturated. 



The tube is calibrated by determining the equilibrium positions of drops of 

 known density. In use, several drops covering the density range in which the 

 experimental drops are expected to lie are introduced in the left side of the 

 tube and an experimental drop on the right side. After 15 min the relative 

 positions are determined with the aid of a cathetometer. As in the falling- 

 drop method, the size of the drops is not critical within reasonable limits 

 (about 0.1 to 10 mm 3 ), but those used must be uniform. The pipette can 

 be somewhat simpler than the type needed for the falling drop, the micro- 

 constriction type being satisfactory [71]. 



As described above, this method gives a precision of only +0.1 per cent of 

 deuterium, but by simultaneously increasing the drop size and decreasing the 

 range of the gradient (using mixtures of densities 0.995 and 1.005, for exam- 

 ple), a precision of 5 parts in 10 6 can be achieved [72]. 



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5. Richards, T. \X ., and J. W. Shipley: /. Am. Chcm. Soc, 36, 1 (1914). 



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